LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 13

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Input Voltage Range
The minimum input voltage is determined by either the
LT3690’s minimum operating voltage of 3.9V (V
3V) or by its maximum duty cycle (see equation in the
Operating Frequency Trade-offs section). The minimum
input voltage due to duty cycle limitation is:
where V
is the minimum switch off-time (210ns). Note that higher
switching frequency will increase the minimum input volt-
age. If a lower dropout voltage is desired, a lower switching
frequency should be used.
The maximum input voltage for LT3690 applications
depends on switching frequency, the absolute maximum
ratings of the V
The LT3690 can operate from continuous input voltages
up to 36V. Input voltage transients of up to 60V are also
safely withstood. However, note that while V
(typically 38V), the LT3690 will stop switching, allowing
the output to fall out of regulation.
For a given application where the switching frequency
and the output voltage are already fixed, the maximum
input voltage that guarantees optimum output voltage
ripple for that application can be found by applying the
following expression:
where V
V
(0.12V at maximum load), V
(0.37V at maximum load), f
(set by RT), and t
(210ns). Note that a higher switching frequency will re-
duce the maximum operating input voltage. Conversely,
a lower switching frequency will be necessary to achieve
optimum operation at high input voltages.
OUT
V
V
IN(MIN)
IN(MAX)
is the output voltage, V
IN(MIN)
IN(MAX)
=
=
1− ƒ
ƒ
is the minimum input voltage, and t
IN
SW
is the maximum operating input voltage,
V
ON(MIN)
and BST pins, and the operating mode.
OUT
V
SW
OUT
• t
+ V
ON(MIN)
• t
+ V
OFF(MIN)
LS
is the minimum switch on-time
LS
SW
SW
− V
LS
is the switching frequency
is the internal switch drop
− V
LS
is the LS switch drop
+ V
LS
+ V
SW
SW
IN
> V
OFF(MIN)
BIAS
OVLO
>
Special attention must be paid when the output is in start-
up, short-circuit, or other overload conditions. In these
cases, the LT3690 tries to bring the output in regulation by
driving current into the output load. During these events,
the inductor peak current might easily reach and even
exceed the maximum current limit of the LT3690, espe-
cially in those cases where the switch already operates at
minimum on-time. The circuitry monitoring the current
through the LS switch prevents the HS switch from turn-
ing on again if the inductor valley current is above I
(5A nominal). In these cases, the inductor peak current is
therefore the maximum current limit of the LT3690 plus
the additional current overshoot during the turn-off delay
due to minimum on-time:
where I
the maximum expected input voltage, L is the inductor
value, t
the output voltage under the overload condition. The part
is robust enough to survive prolonged operation under
these conditions as long as the peak inductor current does
not exceed 9A. Inductor current saturation and excessive
junction temperature may further limit performance.
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to V
switching frequency. In this case, the LT3690 may enter
pulse-skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode,
the output voltage ripple and inductor current ripple will
be higher than in normal operation.
I
L(PEAK)
ON(MIN)
L(PEAK)
= 8A +
is the peak inductor current, V
is the minimum on-time and V
V
IN(MAX)
OVLO
are acceptable regardless of the
L
− V
OUTOL
• t
ON(MIN)
LT3690
IN(MAX)
OUTOL
13
PSDLIM
3690f
is
is